A0A3B6NKR6 (TraesCS6A02G071400, chromosome 6A) is a 360-residue bread wheat member of the plant glucuronokinase family (PANTHER PTHR38710, subfamily SF1) within the GHMP kinase superfamily. It has the GHMP kinase N-terminal and C-terminal domains and the conserved glycine-rich phosphate-binding loop, and is full length relative to the characterised Arabidopsis glucuronokinases GLCAK1 and GLCAK2, with which it shares about 65% identity. By orthology to Arabidopsis GLCAK1 (and the enzyme purified from lily pollen), it is expected to phosphorylate D-glucuronic acid at C1 with ATP to give 1-phospho-alpha-D-glucuronate. This is the step that links the myo-inositol oxygenation pathway, and glucuronic-acid salvage, to UDP-glucuronic acid, the precursor of UDP-xylose, UDP-arabinose and UDP-galacturonic acid for cell-wall polysaccharides. The protein has no predicted signal peptide, transit peptide or transmembrane segment and is expected to be cytosolic. The wheat protein itself has not been characterised experimentally.
Summary: Cytosolic localisation fits a soluble sugar-1-kinase that has no predicted targeting or membrane-spanning sequence. The characterised Arabidopsis ortholog was likewise predicted to be cytosolic. However, the PAINT node behind this IBA (PTN000048555) is seeded by mevalonate kinases (mouse and rat Mvk, yeast ERG12), not by glucuronokinases, and it does not occur in the PTHR38710 tree. The inference therefore rests on the GHMP superfamily, not on the glucuronokinase clade. One experimental report placed a YFP-tagged Arabidopsis paralog GLCAK2 at the plasma membrane, ER and tonoplast in transient tobacco expression; that has not been reproduced and does not fit the protein's lack of membrane features.
Reason: Cytosol is the most plausible compartment for this soluble enzyme. It is consistent with the sequence features and with the prediction made for the characterised Arabidopsis ortholog, so I accept it. The donor node is a mevalonate-kinase node (recorded in the bioinformatics results); this is a weakness of the provenance but not evidence against the location.
Summary: ATP binding is correct for a GHMP kinase. The characterised Arabidopsis ortholog uses ATP as the phosphate donor (Km 0.56 mM), and the wheat protein keeps the GHMP glycine-rich phosphate-binding loop (GLSGSSA).
Reason: ATP binding is true but only part of the catalytic activity. The informative function is glucuronokinase activity, proposed below. ATP binding is not a parent term of that activity, so it is kept as non-core rather than modified.
Summary: This row comes from a UniPathway cross-reference (UPA00057, isopentenyl diphosphate biosynthesis via the mevalonate pathway). That pathway mapping appears to be inherited from mevalonate kinase, another GHMP family. The same PANTHER data list a PAINT node, PTN000048555, seeded by mevalonate kinases, with GO:0019287 and mevalonate kinase activity. A0A3B6NKR6 belongs to the glucuronokinase family PTHR38710, not to the mevalonate kinase family. Its characterised Arabidopsis ortholog is a sugar-1-kinase that is highly specific for D-glucuronic acid. Neither Arabidopsis glucuronokinase carries a mevalonate-pathway annotation in GOA.
Reason: The electronic pathway mapping is demonstrably wrong for this family. Glucuronokinase phosphorylates D-glucuronic acid and acts in nucleotide-sugar metabolism, not in isoprenoid precursor synthesis. Plants make mevalonate with a separate mevalonate kinase from a different GHMP family. The annotation comes from GHMP-superfamily membership rather than from the glucuronokinase clade.
The Arabidopsis enzyme only catalyzed the conversion of d-GlcA into its corresponding sugar-1-phosphate but failed to utilize any other monosaccharide tested in the assays, indicating a very high substrate specificity of AtGlcAK for the sugar d-GlcA.
ISS PMID:19951951 Cloning of Glucuronokinase from Arabidopsis thaliana, the la...
NEW
Summary: A0A3B6NKR6 is in the same PANTHER subfamily (PTHR38710:SF1) as Arabidopsis GLCAK1 (Q93ZC9). GLCAK1 was cloned from peptides of the enzyme purified from lily pollen and shown with recombinant protein to phosphorylate D-glucuronic acid to D-glucuronate-1-phosphate. In the PANTHER tree the plant glucuronokinases form one seed-plant clade with only speciation events between grasses and eudicots, so the wheat protein is an ortholog. It is full length, with both GHMP domains and the phosphate-binding loop. PAINT curators have placed a glucuronokinase-activity IBD at node PTN008680403, seeded by AT3G01640 (2026), but the wheat GOA snapshot does not yet carry the resulting IBA.
Reason: The defining activity of this protein is missing from GOA. Subfamily-level orthology to the biochemically characterised enzyme supports the transfer, and so do the conserved catalytic motifs and the PAINT curators' own glucuronokinase IBD in this family.
ISS PMID:19951951 Cloning of Glucuronokinase from Arabidopsis thaliana, the la...
NEW
Summary: Glucuronokinase catalyses a step of UDP-glucuronate synthesis by the myo-inositol oxygenation route. It makes GlcA-1-phosphate, which UDP-sugar pyrophosphorylase then converts to UDP-GlcA. The gene product therefore does part of the work of the pathway itself; it is not merely a required input. Comparator check: the Arabidopsis UDP-glucose dehydrogenases (UGD2, UGD4), which catalyse the other route to UDP-GlcA, carry GO:0006065 by IDA. Arabidopsis GLCAK1 currently has only TAS inositol-metabolism and cell-wall terms, so this process term is not yet on the characterised ortholog.
Reason: This is a catalytic participation in the pathway, which meets the participation test. It is proposed conservatively at the level of the UDP-GlcA product, not of downstream cell-wall or pollen phenotypes.
It is the last missing enzyme in the myo-inositol oxygenation pathway to UDP-GlcA
Core Functions
ATP-dependent glucuronokinase that phosphorylates D-glucuronic acid at C1 to give 1-phospho-alpha-D-glucuronate in the cytosol. This step of the myo-inositol oxygenation and glucuronate-salvage route to UDP-glucuronic acid supplies nucleotide-sugar precursors for cell-wall polysaccharides. The function is inferred by subfamily-level orthology to biochemically characterised Arabidopsis GLCAK1.
Q: PAINT node PTN000048555, seeded by mevalonate kinases, is attached to PTHR38710 and propagates cytosol to glucuronokinases. Should the cytosol IBD for glucuronokinases be re-anchored on a glucuronokinase node, for example PTN008680403?
Q: Does UniPathway UPA00057, the mevalonate pathway, still map to PTHR38710 members, and can that pathway cross-reference be withdrawn for the glucuronokinase family?
Suggested Experiments
Experiment: Express recombinant A0A3B6NKR6 and assay ATP-dependent phosphorylation of D-glucuronic acid, with D-galacturonic acid, galactose and mevalonate as negative controls. This would confirm the orthology-based assignment in a grass.
External Prediction Reviews
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
A0A3B6NKR6 is a full-length bread wheat member of the plant glucuronokinase subfamily (PANTHER PTHR38710:SF1) of GHMP kinases, with both GHMP kinase domains. By subfamily-level orthology to biochemically characterised Arabidopsis GLCAK1, it is expected to phosphorylate D-glucuronic acid at C1 with ATP. The single ProtNLM2 prediction, the broad process phosphorylation, is correct and novel relative to GOA, which carries only cytosol, ATP binding and an erroneous mevalonate-pathway mapping for this gene.
Prediction method: ProtNLM2 · Version: UniProt 2024_06 pilot
Review rationale: The target is in PANTHER subfamily PTHR38710:SF1 together with Arabidopsis GLCAK1, which was shown with recombinant protein to phosphorylate D-glucuronic acid to D-glucuronate-1-phosphate using ATP (PMID:19951951). The wheat protein keeps both GHMP kinase domains and the glucuronokinase-like signature, so the transfer of ATP-dependent sugar phosphorylation is a justified phylogenetic inference. The gene review proposes glucuronokinase activity (GO:0047940) and UDP-glucuronate biosynthetic process as NEW annotations on the same basis. A wheat study overexpressed the three hexaploid-wheat GlcAK homoeologs in Arabidopsis and reported metabolic phenotypes, but its abstract reports no kinase assay (PMID:36931562). GOA carries no phosphorylation term or descendant for this gene; it has only cytosol, ATP binding and a mevalonate-pathway mapping that the gene review removes. The prediction is therefore novel. It is broad, and a more informative annotation would name the glucuronate substrate.
Supporting Evidence:
PMID:19951951: "AtGlcAK is a novel plant sugar kinase able to phosphorylate d-GlcA to d-GlcA-1-phosphate in the presence of ATP."